A common way of machining orifice holes is punching. It is believed that smaller orifices can be formed with no loss in productivity through the use of laser machining. At least two techniques are believed to be useful for laser machining orifices. One is trepanning or helical drilling, the other is percussion drilling. Percussion drilling is believed to be less desirable due to the less controlled nature of metal heating and expulsion that increases the risk of a non-cylindrical or non-circular orifice. Trepanning, on the other hand, is believed to be more precise as a center hole is initially formed before the formation of the orifice. Helical drilling is similar to trepanning but without the initial formation of a center hole. However, it is believed that neither trepanning nor percussion drilling provides for a desired formation of entry and exit geometry in the orifices.
With laser machining, debris (recast material, melted material and soot from machining) can be formed both immediately at the perimeter of the orifice and on the surface some distance from it on both the entry and exit sides of the orifice. This leads to an orifice that is not uniform dimensionally with respect to, such as, the diameter, exit or entry geometry of the orifices.
The present invention provides for protection of a workpiece during laser machining of an aperture through the workpiece so that the surface of the workpiece and areas surrounding the aperture are substantially free of debris produced by the laser machining process. In particular, a preferred embodiment of the present invention provides for a thin metallic plate. The metallic plate comprises first and second surfaces of the metallic plate being spaced apart at a distance of approximately 50 to 300 microns. The metallic plate includes at least one orifice extending between the first and second surfaces. The at least one orifice is formed, in part, by forming a carbonizable polyimide coating on one of the first and second surfaces; drilling the orifices through the coating and the first and second surfaces so that a taper of the at least one orifice is less than 10% and ellipticity of each orifice is less than 10%; and removing the coating. The taper is defined as a difference between average entry diameter and average exit diameter and divided by thickness of the work piece multiplied by 100, and the ellipticity is defined as a difference between the major diameter and minor diameter multiplied by two and the result divided by the sum of the major diameter and minor diameter multiplied by 100.
The present invention also provides a method of forming an aperture on a thin metallic workpiece by a laser. The method can be achieved, in part, by forming a carbonizable polymer coating over at least one surface of the workpiece; machining at least one aperture between the at least one surface and another surface of the workpiece via a laser so that so that, upon removal of the polymer, a taper of the at least one orifice is less than 10% and ellipticity of each orifice is less than 10%, the taper being a difference between average entry diameter and average exit diameter and divided by thickness of the work piece times 100, the ellipticity being a difference between the major diameter and minor diameter multiplied by two and the result divided by the sum of the major diameter and minor diameter times 100; and removing the carbonizable polymer from the workpiece.